The present invention relates to an image density control method for an image forming apparatus of the type forming a latent image representative of a document image on a photoconductive element and developing the latent image to produce a toner image by an electrophotographic procedure.
A predominant type of copier or similar image forming apparatus which is implemented by an electrophotographic procedure uses a two component developer, i.e. the mixture of a toner and a carrier. In this type of copier, for example, as the toner is consumed by the repetitive copying process, the toner concentration in the developer is sequentially reduced to in turn lower the density of the resultant toner image. It has been customary, therefore, to supply a supplementary amount of toner to the developer to maintain the density of the developed image constant. In an automatic density control mode, a desired or target image density is associated with the density of a document image which is sensed by a document density sensor. On the other hand, in a manual density control mode, the target density is associated with a particular image notch manually selected on an operation board of the copier. Generally, the first to seventh notches are available with a copier, and the image density decreases with the increase in the notch number. For this kind of image density control, use may be made of a reference density pattern having a reference density, as well known in the art. Specifically, a latent image representative of the reference density pattern is formed on a photoconductive element and then developed by the toner, every time a predetermined number of copies are produced. Then, an image density sensor (sometimes referred to as a P sensor) optically senses the density of the resultant toner image. The sensed image density is fed back to a toner supply section of a developing device included in the copier to supply an adequate amount of toner, whereby the image density is maintained constant. This method determines a change in the toner concentration of the developer, i.e., a change in the proportion of the toner to the carrier in terms of a change in the density of the toner image of the reference pattern formed on the photoconductive element, thereby controlling the toner concentration of the developer. While a reflection from the reference density pattern is weak when the toner concentration is high, it becomes intense as the toner density decreases. The reference voltage of the image density sensor or P sensor (surface potential of the photoconductive element developed by an eraser) is usually selected to be 4 V. Then, when the output of the sensor associated with the reference density pattern is higher than 0.5 V which is one-eighth of 4 V and representative of an adequate toner concentration, the toner is determined to be short and, therefore, it is supplied. When the output of the sensor is lower than 0.5 V, the toner is determined to be sufficient and not supplied at all.
The surface potential of the photoconductive element sequentially increases with the number of copies produced. In addition, mirrors and a glass platen incoporated in the machine are contaminated by toner particles and silicone oil which is used to fix a toner image on a paper sheet and vaporizes, resulting in the decrease in the amount of light and, therefore, in the increase in the potential of light areas. By such an increase in the surface potential of the photoconductive element, the image density adequately adjusted at first is shifted to the dark side with the result that the background is disturbed.
To eliminate the drawback of the toner supply control scheme stated above, there has also been proposed a method which substantially variably controls the developing ability by controlling the total current to be fed to a charger which charges the photoconductive element, the bias voltage for development to be applied to a developing sleeve of the developing device, the voltage to be applied to a lamp of optics, etc., or by relying on the user's manual correction. Such an approach is also successful in setting up a desired image notch and disclosed in, for example, Japanese Patent Laid-Open Publication (Kokai) Nos. 61-128269, 62-280871 and 54-134635, and U.S. Pat. No. 4,618,248.
A photoconductive element for use in an electrophotographic copier or similar image forming apparatus is often implemented by As.sub.2 Se.sub.3 which is an inorganic compound of selenium and a small amount of arsenic. This kind of photoconductive element has the highest sensititivity. The surface of As.sub.2 Se.sub.3 is coupled with oxygen existing in the air to form an AsO (arsenic oxide) layer, whereby a charge is retained on the photoconductive element. This brings about a problem that the charge retaining ability depends on the condition of the AsO layer. Since an As.sub.2 Se.sub.3 photoconductive element has hardly any charge retaining ability just after evaporation, it is left in the dark until the charge retaining ability reaches saturation. However, about three to six months are needed for the charge retaining ability to reach saturation. This results in the need for a considerable amount of stock and, therefore, in low productivity. To accelerate such a procedure, i.e., to reduce the period of time over which the photoconductive element should be left in the dark, the element just undergone evaporation may be loaded in a copier, then run with paper sheets for a test for about five to fifteen minutes, and then left in the dark. In practice, however, a copier is put on the market without its photoconductive element being left in the dark for such a sufficient period of time, and it is actually operated before the element attains the expected charge retaining ability. While a serviceman usually tests a new copier for about 5 minutes on the delivery of the machine to a user in order to provide it with as great a charge retaining ability as possible, such a measure is not satisfactory. With a copier having an As.sub.2 Se.sub.3 photoconductive element, it usually occurs that after the installation of the copier the potential (background potential) of the element increases by about 90 V when about 1,000 copies are produced, i.e., on the lapse of about one to three months. Such an increase in the potential shifts the entire image to the dark side and thereby contaminates the background, often constituting the cause of serviceman call.
Optics built in a copier is generally made up of a glass platen, mirrors, a lens, a dust glass, and an arrangement for cooling the entire optics. When various contaminants such as dust floating in the air, the vapor of oil filling the machine and toner particles deposit on the mirrors and other components of the optics, the transmittance and/or reflectance of the entire optics is lowered to reduce the quantity of light available for imagewise exposure. Especially, the prior art automatic density type control method does not take account of the deposition of such contaminants, i.e., the decrement of the amount of light, so that the entire image is shifted to the dark side. For example, assuming that maintenance cycle a copier is about 80,000 copies, the decrement of the quantity of light corresponds to about 100 V to 200 V in terms of the potential of the photoconductive element. Hence, the density is brought out of the automatic control range, constituting another cause of serviceman call. The shift of the potential of the photoconductive element to the dark side as stated above means that the background potential of the element is changed to contaminate the background.
The conventional image density control of the kind using an image density sensor or P sensor does not give any consideration to the problems discussed above, i.e., it simply controls toner supply in such a manner as to maintain the developing ability constant. Hence, the image density is prevented from matching a selected image notch. This is also true with the alternative approach shown and described in any of the previously mentioned Laid-Open Publications. Specifically, the alternative approach replaces one variable factor capable of changing the developing ability with another variable factor when the former reaches a predetermined value. However, it does not detect a change in the density of the background and, therefore, cannot automatically deal with the background contamination ascribable to the shift of the potential of the photoconductive element to the dark side.